Standard reduction potentials: $\mathrm{Mg}{}^{2+}/\mathrm{Mg}=-2.37$, $\mathrm{Zn}{}^{2+}/\mathrm{Zn}=-0.76$, $\mathrm{Fe}{}^{2+}/\mathrm{Fe}=-0.44$, $\mathrm{Cu}{}^{2+}/\mathrm{Cu}=+0.34\mathrm{V}$. Which metal cannot liberate $\mathrm{H}{}_2$ from $1\mathrm{M}$ HCl?

A metal displaces hydrogen from acid if $E^{\circ }(\mathrm{M}{}^{n+}/\mathrm{M})$ is below $E^{\circ }(\mathrm{H}{}^{+}/\mathrm{H}{}_2)=0$, because then $E_{\text{cell}}^{\circ }=0-E^{\circ }(\mathrm{M}{}^{n+}/\mathrm{M})>0$. Mg, Zn and Fe have negative values and react. Cu ($+0.34\mathrm{V}$) lies above hydrogen in the series and does not liberate $\mathrm{H}{}_2$.